A power transmission system includes a power transmission apparatus and a power receiving apparatus. The power transmission apparatus uses magnetic field coupling or electric field coupling to transmit power to the power receiving apparatus wirelessly. The power receiving apparatus supplies the transmitted power to a load such as a secondary battery of a cellular phone or a laptop computer, for example, and charges the secondary battery. In such a power transmission system, it is necessary for the power transmission apparatus to monitor the type, status, and so on of the power receiving apparatus. Accordingly, the power transmission apparatus and the power receiving apparatus are provided with communication functions that enable the power transmission apparatus and the power receiving apparatus to communicate with each other.
A system using load modulation, for example, is used as a system of communication between the power transmission apparatus and the power receiving apparatus. In this system, signals of “0” and “1” are sent from the power receiving apparatus to the power transmission apparatus by varying a load impedance on the power receiving apparatus side as viewed from the power transmission apparatus side. However, in the case where there is a load variation, there are cases where the power transmission apparatus cannot determine whether the load variation is due to communication or whether the load variation is due to the status of the load of the power receiving apparatus. For example, in the case where the load of the power receiving apparatus is a laptop computer being used by a user, if the CPU load in the laptop computer increases suddenly and the consumed power rises momentarily while power is being transmitted, the load impedance will momentarily decrease as seen from the power transmission apparatus side. In this case, the load variation is not a load variation caused by communication, and thus there is a risk of erroneous information being obtained on the power transmission apparatus side.
Patent Document 1 discloses a system, in which power transmission and communication can be carried out between a power transmission apparatus and a power receiving apparatus, where power supply and communication are not carried out simultaneously. In this system, each apparatus uses a resonating element (a coil) for power transmission as a communication antenna. A switching circuit is provided between the communication unit and the resonating element, and the switch is switched off during the supply of power and on during communication. This ensures that power supply and communication are not carried out simultaneously.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2011-29799.
However, the system disclosed in Patent Document 1 has the following problem. For example, in the case where the load of the power receiving apparatus is a laptop computer battery and the remaining charge in the battery is approximately 0, if, as in Patent Document 1, the switch is turned off for communication and the supply of power to the battery is turned off while the computer is running and power is being supplied to the battery, problems such as the computer having insufficient power and not operating correctly, the computer forcefully being shut down during processing, or the like may arise. In this manner, there is a risk that unanticipated problems will arise in the load of the power receiving apparatus if the load is cut off during communication and the supply of power to the load is turned off without consideration for the status of the load.